A crystal has a repeating pattern of atoms, ions, or molecules with fixed distances between constituent parts. Photonic band gap crystals, referred to as photonic band crystals (PBCs) or simply photonic crystals, are characterized by materials with different refractive indices generally periodically spaced in one or more dimensions. The periodic structure and the properties of the selected materials creates a band gap, a range of wavelengths of electromagnetic radiation that are reflected and cannot pass through the photonic crystal.
Photonic crystals have many applications including as filters, low-loss waveguides, high “Q” resonators, antennae, etc. For a filter, for example, incident light with a wavelength in the photonic band gap would be strongly reflected, producing a filter stop band, while light with a wavelength outside the photonic band gap would be transmitted through the photonic crystal.